Pearce et al. U.S. Pat. No. 5,101,688 discloses a transmission system for an agricultural tractor. In a system of the type disclosed in the patent it has been conventional to employ a fixed pressure-versus-time relationship in controlling the clutches during shifting. During shuttle shifting operations, that is, shifts between forward and reverse gears, this results in significant variations in acceleration depending on the weight of the tractor in which the transmission is installed, and the load the tractor is pulling or pushing. For automatic clutch engagement when starting from rest, a compromise pressure versus time relationship must be used to prevent excessive clutch slippage under high load conditions, yet provide a reasonably smooth start-up with no load.